This invention relates to an electron lens which can reduce both radial and spiral distortions in a transmission electron microscope.
The aberrations which must be carefully avoided in the design of a projector lens for an electron microscope include radial distortion, spiral distortion, chromatic aberration of rotation, and chromatic aberration of magnification. The radial (isotropic) distortion, which presents a more important problem than any other kind of aberration, can be substantially eliminated in a low magnification range, where the projector lens is magnetized with less lens excitation, by virtue of the so-called distortion-free system (U.S. Pat. No. 3,188,465) in which a higher barrel type distortion is created in an intermediate lens to cancel a pincushion type distortion caused by the projector lens. In a medium magnification range where a fixed amount of electric current is applied to the projector lens, however, no such cancelling can be expected since the pincushion type distortion caused by the projector lens is overwhelmingly greater than the barrel type distortion caused by the intermediate lens, and there is inevitably a distortion of, say, 1 to 2% along the circumference of a circle having a diameter of 100 mm on a fluorescent screen.
There has hitherto been no effective method for eliminating spiral (anisotropic) distortion. All that has been done in the past was to make the distance between the projector lens and film surface as long as possible, and utilize only the electron beams passing in the vicinity of the central axis, so that the aberration may not easily be noticeable. According to this method, however, it is difficult to reduce the aberration to a level of 2% or less, because it is not adapted to basically reduce the aberration, and also because certain spatial restrictions are imposed on the apparatus which is available for carrying out the method.
It has recently been proposed to use a projector lens a lens equipped with three magnetic pole pieces defining two gaps of opposite excitation, and this lens has been found capable of eliminating radial distortion completely. It has also been found that this lens can reduce spiral distortion to a level not possible by any other type lens. The lens, however, does not eliminate spiral distortion completely, and if it is desired to achieve a low spiral distortion of 1% or less, there is no alternative but to use the lens in a range of lens excitation in which radial distortion increases.
FIG. 1 is a view showing schematically the electron lens proposed prior to this invention. In the figure, two excitation coils 1 and 2, which are connected in series and supplied with the current (I) from a lens power supply 3, are enveloped by a ferromagnetic yoke 4 and non-ferromagnetic spacers 5 and 6. Inside the yoke 4, the upper pole piece 7, middle pole piece 8 and lower pole piece 9 and their non-ferromagnetic spacers 10 and 11 are installed. The shape of the lens is nearly symmetrical with respect to the center of the middle pole piece 8. The upper d1, middle d2, and lower d3 pole bore diameters are all 3 mm, and the first gap length S1 between the upper and middle pole pieces is equal to the second gap length S2 between the middle and lower pole pieces. The turn number (N) of each lens coil, 1 and 2, is the same and the winding direction of each coil is determined so that polarity of the magnetic field appearing in the first and second gaps is opposite to each other and the magnetic field appearing in the first and second gaps is generated by the same excitation intensity.
FIG. 2 shows the focal length fp (mm), radial distortion .DELTA.X/X (%) and spiral distortion .DELTA.Y/X(%) of the lens shown in FIG. 1 in relation to the excitation (magnetomotive force) NI (ampere turns), using the thickness t of the middle pole piece 2 as a parameter. The graph shown refers to a lens having an equal bore diameter d1, d2 or d3 of 3 mm, and equal pole gap length S1 or S2 of 2.25 mm, and a middle pole piece thickness t of 1 mm and 2 mm. The graph shown is measured at a point apart from the optical axis (r=50 mm) on a fluorescent screen (or film) which is located under the projector lens by L=386.5 mm. Further, the graph shown is obtained under the condition in which accelerating voltage of the electron beam equals 100 KV. In the event that accelerating voltage does not equal 100 KV, the following conversion equation is established ##EQU1## where: V*=Accelerating voltage (V) of the electron beam
(NI)=Value of NI (ampere turns) in the case that accelerating voltage of the electron beam equal V*. PA0 .sup.(NI) 100 KV=Value of NI (ampere turns) in FIG. 2.
As is noted from FIG. 2, the focal length fp shows a minimum value at the excitation NI of 2,200 and 1,800 AT (ampere turns) when the middle pole thickness t is 1 mm and 2 mm, respectively. The minimum values of the focal length are 3.8 mm, and 4.6 mm when the thickness t is 1 mm and 2 mm, respectively. It is, thus, noted that a lens having a smaller thickness t of the middle pole piece has a smaller minimum value of focal length. The radial distortion .DELTA.X/X shows a positive value (pincushion type) on the low excitation side and a negative value (barrel type) on the high excitation side. The excitation value at which the radial distortion becomes zero is substantially equal to that at which the focal length fp shows the minimum value. On the other hand, spiral distortion .DELTA.Y/X is very small on the low excitation side, but shows a sharp increase with an increase in the excitation. And there is no situation in which the spiral distortion becomes zero. Accordingly, if the radial distortion .DELTA.X/X is zero or at a very low level, the spiral distortion .DELTA.Y/X cannot be reduced to zero.
If different amounts of excitation are applied to form magnetic fields in the two gaps between the magnetic pole pieces in the lens constructed as shown in FIG. 1, it is possible to eliminate any spiral distortion at certain amounts of excitation, but other defects, such as chromatic aberration of rotation, arise and prevent effective utilization of the features of a lens having three magnetic pole pieces.